1. Field of the Invention
The present invention relates to a Diesel engine having a subchamber or a vortex chamber in the cylinder head and provided with an improved exhaust gas recirculating system.
2. Description of the Prior Art
In some conventional Diesel engines provided with a subchamber or a vortex chamber in the cylinder head, an exhaust gas recirculating system (hereinafter EGR) substantially as shown in FIG. 1 has been provided in order to decrease NOx products produced in the combustion stroke.
In such a conventional Diesel engine, an EGR passageway 2 is connected to an intake manifold 1 and a mixed gas of intake air and a recirculating exhaust gas is charged to a main combustion chamber 5 through an intake port 3 and an intake valve 4. Mist fuel is ejected from a fuel injection valve 9 into a vortex chamber 8 formed in a cylinder head 7 at the proximity of upper dead point in the compression stroke of a piston 6 and thus ejected fuel is further compressed and ignited in the mixed gas.
This system is designed to suppress formation of NOx by lowering the highest temperature of combustion with a substantial quantity of substantially inert EGR gas having high thermal capacity.
The combustion phenomenon of such a subchamber or vortex chamber type Diesel engine is mainly based upon diffusion combustion in which the flame is ejected into the main combustion chamber 5 together with unburned mist fuel with the increase of pressure through a nozzle or throat passage 10 after its ignition in the vortex chamber 8.
In such type of diffusion combustion, the propagation of the flame is not effected in the area where the mist fuel does not exist. This is quite different from the premixed combustion in a gasoline engine in which a mixed gas is previously filled up in the combustion chamber.
For decreasing the production of the amount of NOx below a predetermined value by such an EGR system, it is necessary for the EGR gas to be present in the combustible region of the engine where the ejected mist fuel is located.
In a conventional system, the EGR gas is charged through the main combustion chamber 5. Accordingly, if the EGR gas concentration is to be maintained at a sufficiently high value in the vortex chamber 8 and in the region where the diffused mist fuel exists, there results an introduction of excess amount of the EGR gas over the whole area of the main combustion chamber 5.
Under certain condition of driving a vehicle equipped with such an engine, the rate of introduction of the EGR gas may reach up to 50% to 80%. The increase of the EGR gas, which may prevent perfect combustion, to such an extent results in an undesired increase of HC.CO even though the amount of NOx can be decreased.
It has been proposed to provide a thermal reactor or an oxidizing catalyst in the exhaust system of an engine for suppressing such increase of production of HC. However, in a Diesel engine, which has a lower exhaust gas temperature when compared with a gasoline engine, the reaction efficiency of such proposed systems is not sufficient. Accordingly, the conventional EGR system has difficulty in solving the problem of production of unburned HC.CO.